You are trying to create a popupInfo for a graphic in a graphics layer and not in feature layer. Hence, you cannot control the order of the fields. If you want to maintain the order than you should set desired AGSPopupFieldInfo's array in AGSPopupInfo::fieldInfos.

Like this..

1. Create a new mutable array.2. Read each record of popupInfo.fieldInfos and add in the new array in desired order.3. Set new mutable array to popupInfo.fieldInfos